We are writing to express our shock and dismay at the addition to the Guardian's US commentary team of a man who has openly called for the killing of people on humanitarian missions to Palestine, people who have included the Pulitzer-prize-winning author Alice Walker.

The extreme views of your new freelance contributor Joshua Treviño, whose columns will appear on your Guardian US website from tomorrow, are no secret. In 2011, he used Twitter to urge the Israeli army to murder unarmed US citizens who were preparing to sail from Greece on a flotilla to Gaza. Treviño tweeted: "Dear IDF: If you end up shooting any Americans on the new Gaza flotilla – well, most Americans are cool with that. Including me." He also backed a tweet which called for the sinking of the flotilla, which would have endangered the lives of all on board, and likened this peaceful mission to a "Nazi convoy".

In what way does publishing a man who clearly has no regard for the rule of law, and who advocates the killing of his fellow citizens by a foreign army, enhance the Guardian's reputation as a serious newspaper? The extremist views of people like Treviño, who have no hesitation in wishing death upon those who disagree with them, can be found on countless sensationalist, racist and hate-speech websites. They have no place in a reputable publication.

Moreover, Treviño is hardly without vested interests. He served on the board of the pro-Israel group Act for Israel, and was listed on its website as being "a staunch digital advocate of Israel". This former speechwriter for George W Bush will no doubt be bringing his one-sided political views to the Guardian and using it as a platform for his propaganda. It is a sad day for responsible and impartial journalism when the opinions of such a man are sought as an "important perspective" (the words of Janine Gibson, editor-in-chief of the Guardian US) by a supposedly progressive publication. Sarah Colborne Director, Palestine Solidarity CampaignRoger Lloyd Pack ActorKika Markham ActorBruce Kent Vice-president, CNDLindsey German Stop the WarDaud Abdullah Middle East MonitorZahir Birawi Palestinian Forum of BritainDiana Neslen Jews for Justice for PalestiniansChris Rose Amos TrustShenaz Bunglawala iEngageBaroness Jenny TongeJeremy Corbyn MPJohn AustinProfessor Nur Masalha Soas*Professor Ilan Pappe Exeter University*Dr Ghada Karmi Exeter University*Professor Jonathan Rosenhead LSE*Professor Kamel Hawwash University of Birmingham*Professor Haim Bresheeth Professor Antoine Zahlan (retd) American University of Beirut*Professor Steven Rose Open University*Professor Hilary Rose LSE*Dr Les Levidow Open University*Canon Garth Hewitt St George's Cathedral, JerusalemAhdaf Soueif Author and journalistVictoria Brittain Author and journalistAbe Hayeem Chair, Architects and Planners for Justice, Palestine(*all writing in a personal capacity)